1. Field of the Invention
The present invention relates to a surface acoustic wave filter device. More particularly, the present invention relates to a surface acoustic wave filter device including a surface acoustic wave filter chip flip-chip mounted on a wiring board.
2. Description of the Related Art
In the past, a variety of surface acoustic wave filter devices using surface acoustic waves have been proposed, for example, in the following Japanese Unexamined Patent Application Publication No. 2002-141771 and so forth as a band pass filter or the like mounted on or in an RF (Radio Frequency) circuit of a communication device, such as a cellular phone.
FIG. 14 is a schematic cross-sectional view of a surface acoustic wave filter device described in Japanese Unexamined Patent Application Publication No. 2002-141771. As illustrated in FIG. 14, a surface acoustic wave filter device 100 includes a package 101 and a surface acoustic wave filter device 102 disposed in the package 101. The package 101 includes a base substrate 101a, side walls 101b, and a cap member 101c. The surface acoustic wave filter device 102 is flip-chip mounted on the base substrate 101a. 
FIG. 15 is a schematic plan view of the surface acoustic wave filter device 102. FIG. 16 is a schematic circuit diagram of the surface acoustic wave filter device 100. FIG. 17 is a schematic plan view of the base substrate 101a. As illustrated in FIG. 15, the surface acoustic wave filter device 102 includes a piezoelectric substrate 102a and a ladder surface acoustic wave filter unit 102b formed on the piezoelectric substrate 102a. As illustrated in FIG. 16, the surface acoustic wave filter device 100 includes series arm resonators 104a and 104b connected in series in a series arm 103 (see FIG. 16) and parallel arm resonators 105a to 105c provided in parallel arms 106a to 106c connecting the series arm 103 and the ground potential. Between the parallel arm resonators 105a to 105c and the ground potential, inductors L101 to L103 (see FIG. 16) are provided. The series arm 103 is provided with inductors L104 and L105. The series arm resonators 104a and 104b and the parallel arm resonators 105a to 105c are formed on the piezoelectric substrate 102a, as illustrated in FIG. 15. Meanwhile, the inductors L101 to L105 are formed on the base substrate 101a, as illustrated in FIG. 17. Specifically, the inductors L101 to L105 are formed by wiring lines 107a to 107e formed on the base substrate 101a. The surface acoustic wave filter device 102 is flip-chip mounted on the base substrate 101a, and thus a surface of the base substrate 101a is a die-attach surface.
Meanwhile, it has been known in the past that it is possible in a ladder surface acoustic wave filter to improve filter characteristics by connecting inductors in series or parallel with series arm resonators or parallel arm resonators. Specifically, it is possible to widen the pass band and increase the attenuation near the pass band. In view of this, the inductors L101 to L105 are provided in, for example, the surface acoustic wave filter device 100 described in the above-described Japanese Unexamined Patent Application Publication No. 2002-141771 in terms of improvement of filter characteristics.
The inductors may be formed by chip inductors mounted on the base substrate or the like. In terms of a reduction in size of the surface acoustic wave filter device, however, it is preferred that the inductors are formed by wiring lines formed on the base substrate. As an area in the base substrate for forming the wiring lines forming the inductors, the interior of the base substrate or the die-attach surface of the base substrate is conceivable. If the wiring lines forming the inductors are formed in the interior of the base substrate, however, the distance between the wiring lines forming the inductors and the ground potential is reduced. It is therefore difficult to form inductors having a large inductance value. Accordingly, it is preferred that the wiring lines forming the inductors are formed on the die-attach surface of the base substrate, which is located at a position that is farthest from the ground potential.
If the wiring lines forming the inductors are formed on the die-attach surface of the base substrate, however, electromagnetic field coupling or capacitive coupling may occur between the wiring lines forming the inductors and the resonators formed on the piezoelectric substrate and degrade the filter characteristics. Therefore, in the surface acoustic wave filter device 100, for example, the wiring lines 107a to 107e forming the inductors L101 to L105 are formed in a peripheral portion of the surface of the base substrate 101a so as not to face the series arm resonators 104a and 104b and the parallel arm resonators 105a to 105c on the piezoelectric substrate 102a, as illustrated in FIGS. 15 to 17. In the surface acoustic wave filter device 100, therefore, it is necessary to increase the size of the base substrate 101a to secure the area for providing the wiring lines 107a to 107e forming the inductors L101 to L105, and there is an issue of an increase in size of the surface acoustic wave filter device. Further, particularly in a small-sized surface acoustic wave filter device, such as a CSP (Chip Size Package) surface acoustic wave filter device, it is difficult to secure the area for providing the wiring lines forming the inductors in the peripheral portion of the surface of the base substrate.